As early as 1960, U.S. Pat. No. 2,940,920 disclosed the separation of deasphalting solvent from deasphalted oil under supercritical conditions for the solvent (supercritical is intended to mean a pressure higher than the critical pressure and a temperature higher than the critical temperature; the critical temperature is the maximum temperature at which at least a part of the fluid can be liquefied by isothermal compression; the critical pressure is the maximum pressure at which condensation or boiling occurs by temperature variation at constant pressure).
For example, this patent specification, at column 18, line 20, discloses the separation of normal pentane, on the one hand, from deasphalted and deresined oil, on the other hand, at a temperature of 215.degree. C. and at a pressure of 37.5 bars, whereas the critical temperature and pressure of pentane are respectively 196.degree. C. and 33 bars.
It can be observed that this patent also discloses deresining of deasphalted oil under supercritical conditions for the solvent.
U.S. Pat. No. 4,305,814 extends this technique to the solvent-asphalt separation and to the solvent-resin separation: the oil to be treated is contacted, in a first settler, with solvent and an oil-resin-solvent mixture is separated from asphalt which still contains some solvent. In a first step, the asphalt+solvent mixture is brought to supercritical conditions for the solvent and there is separated, on the one hand, substantially pure solvent and, on the other hand, asphalt containing solvent traces which will then be stripped with steam. The oil-resin solvent mixture is so heated as the bring it to conditions which may be slightly supercritical; a solvent-oil mixture is separated from resin still containing solvent. In a second step, the resin+solvent mixture is heated to supercritical conditions for the solvent whereby additional solvent can be recovered before stripping the last solvent traces from the resin fraction. Finally, the oil-solvent mixture is heated so as to bring it to supercritical conditions before separating most of the solvent from the oil.
In the mentioned example, the deasphalting solvent is penetane, the conditions in the first step are 238.degree. C., 46.4 bars and the conditions in the second step are 240.degree. C., 46 bars.
U.S. Pat. No. 4,502,944 discloses deasphalting and deresining in a single step with demixing to 3 liquid phases, at a temperature of about 15.degree.-20.degree. C. lower than the solvent critical temperature and at a pressure about 20 bars higher than the critical pressure. The lighter phase, consisting of a solvent-oil mixture, is passed through an exchanger and through a furnace before entering a supercritical separator at a pressure equivalent to that of the first separator (with the approximation of the pressure drops) and at a temperature higher than the critical temperature of the solvent, not specified. Then the separated solvent passes through the above-mentioned exchanger and heats the oil-solvent mixture.
In the three above-mentioned patents, to steps conducted under supercritical conditions are destined to separate the deasphalted oil from the deasphalting solvent.
This is diagrammatically expressed as follows:
______________________________________ "cold" extraction step "hot" step of supercritical by undercritical solvent separation liquid solvent ______________________________________
It is thus desired, in a first type of process according to these documents, to minimize the amount of matter stripped with the supercritical fluid. In other documents, for example in U.S. Pat. Nos. 4,201,660, 4,478,705, 4,363,717, 4,341,619, 4,482453, 4,349,415 and 4,354,922, the operating principle is of a second type which can be considered as the inverse: it is first desired to strip, from a substance to be refined, a class of components which will dissolve into the supercritical phase and then, during a second expansion and/or heating stage, to separate the solvent from the stripped extract, this being diagrammatically expressed as follows:
______________________________________ 1 - High pressure 2 - Low pressure separation supercritical extraction step under critical or step slightly supercritical conditions ______________________________________
or by succession of:
1--High pressure supercritical extraction PA1 2--Release of a part of the stripper matter by heating (supercritical conditions) PA1 3--Undercritical low pressure separation. PA1 first, the heat of the first recovered solvent phase, PA1 then, the heat of the second extract phase of increased oil content.
In this second type of scheme, the object is to maximize the amount of matter stripped with the supercritical fluid.